In this we condemn the theory which refuses to recognise unconscious purpose as present in each individual case of instinctive action. For he who maintains instinct to be the result of a mechanism of mind, must suppose a special and constant mechanism for each variation and modification of the instinct in accordance with exterior circumstances, {97} that is to say, a new string giving a note with a new tone must be inserted, and this would involve the mechanism in endless complication. But the fact that the purpose is constant notwithstanding all manner of variation in the means chosen by the instinct, proves that there is no necessity for the supposition of such an elaborate mental mechanism—the presence of an unconscious purpose being sufficient to explain the facts. The purpose of the bird, for example, that has laid her eggs is constant, and consists in the desire to bring her young to maturity. When the temperature of the air is insufficient to effect this, she sits upon her eggs, and only intermits her sittings in the warmest countries; the mammal, on the other hand, attains the fulfilment of its instinctive purpose without any co-operation on its own part. In warm climates many birds only sit by night, and small exotic birds that have built in aviaries kept at a high temperature sit little upon their eggs or not at all. How inconceivable is the supposition of a mechanism that impels the bird to sit as soon as the temperature falls below a certain height! How clear and simple, on the other hand, is the view that there is an unconscious purpose constraining the volition of the bird to the use of the fitting means, of which process, however, only the last link, that is to say, the will immediately preceding the action falls within the consciousness of the bird! In South Africa the sparrow surrounds her nest with thorns as a defence against apes and serpents. The eggs of the cuckoo, as regards size, colour, and marking, invariably resemble those of the birds in whose nests she lays. Sylvia _ruja_, for example, lays a white egg with violet spots; _Sylvia hippolais_, a red one with black spots; _Regulus ignicapellus_, a cloudy red; but the cuckoo’s egg is in each case so deceptive an imitation of its model, that it can hardly be distinguished except by the structure of its shell.